I don't think there's a sum-it-up thread on this subject. Also, I'd like some other folks to help me out if they would by discussing in DETAIL how Chameleon works.

1. Chameleon (sometimes also called "Shadow" in the game). Theoretically, if you're at 100% Chameleon, other characters can't detect you at ALL, no matter what you do -- stab them, steal something, anything. In practice, however, I've gone to 100% Chameleon and still had them detect and talk to me, or see me stealing. This has happened often enough that I'm almost certain it's not simply due to me making arithmetic errors.

Chameleon is a hard spell to cast. Normally you can only get up to 100% chameleon for a few seconds at a time, so plan ahead exactly what actions you'll take and then go for it. It is possible, however, to make a bunch of CE items that add up to 100% chameleon. And it's easy (if you find enchanting easy at all) to make CWU items that give 100% Chameleon for a few seconds, perhaps combined with Telekinesis, as in my Pants of Thievery or Thiefs Locket.

2. Telekinesis. Morrowind operates on the ostrich principle; if you can't see somebody, they can't see you either. It doesn't matter if you steal something in front of their nose; if they can't see YOU, they let you get away with it. So a great way to steal is stand where you can't be seen, use telekinesis to reach over, and take something, whether that's just grabbing an item or emptying a crate or unlocking and then emptying a closet.

Using the standard 25 foot Telekinesis spell, you can walk through the door of the Balmora alchemist, and, WITHOUT TAKING ANOTHER STEP, loot a bunch of stuff including a Grandmaster Mortar/Pestle. You also can go to the Eastern Guard Tower at the Northeast end of town, jump on a nightstand, and grab the Sword of White Woe that's on top of the closet. At least, I was able to when I tried it. (Jumping on the nightstand was to get out of the guard's sight.)

3. Open. Obviously, this is the most important magical aid to thievery. I have stolen tens of thousands of gold pieces worth of stuff while my Security skill was still 5. You can buy Ondusi's Open Lock easily, and have custom spells made for, say, Open 100 and Open 80. Buying the Amulet of Opening, which opens up to 20, will save you some magicka (which is precious early in the game).

4. Invisibility. I've written about this in other threads. If you get caught stealing, Invisibility is very helpful both for the immediate getaway and then for avoiding guards while you get to a place to buy off your bounty. Levitate satifies the same functions, although not necessarily quite as well. Invisibility is also good for making sure folks aren't looking at you when you start to Sneak or do other nefarious things. Not seeing you may also affect what path they take when they are patrolling.

5. Teleportation -- Recall or the Interventions. Crucial for a thief, both in case you get overloaded with loot and in case you have to skedaddle quickly. I just cast Mark in front of Creeper and Recall back to him every time I want to lighten up my inventory.

6. Fenrick's Doorjam. Nobody talks about this one. But it's often desirable to sleep in a strange bed, e.g. to restore your magicka. 19 times out of 20, if you close the door behind you, nobody will come in. The 20th, you'll wish you'd used the Doorjam spell. If you have an Amulet of Opening, it won't cost you any magicka to leave the room after you wake up.

Locking a door behind you is also good if a room is empty and you want to keep it that way.

While the practical need for this spell is debatable, I also think it enhances roleplaying. Would you sleep in a room you'd just cleaned out, with lots of people running around, unless you locked the door behind you?

7. Command Humanoid. I haven't used this much, but it's one obvious way to get rid of witnesses before you steal something.

8. Detect Key, Detect Enchantment. I've never used these. Has anybody found them helpful?

9. Levitate. Besides avoiding guards, ala invisibility, levitate lets you get around (especially to back doors on the second story, although the game's characters overstate the importance of that) and look around. Most important, it lets you get to the top floor of Telvanni towers, and therefore in particular to steal the CotSH in Tel Fyr. Plus it can sometimes help you get just the right angle so that a guard doesn't see you.

10. Paralyze. If you steal something, and that causes a fight, but you don't want to kill the person, Paralyzing could also keep them out of your way until you leave. It's best to lure them first to a remote part of the building, so that they don't see you steal more and increase your bounty.

11. There is, of course, lots of magic that helps you kill people, and some thieves kill people. I won't get into that in this thread.

12. Fortify Luck. Fortify Luck helps with everything. Lockpicking and trap disarmament should be no problem. Also, it may be optimal to Fortify your Luck before casting a difficult spell, to increase its probability of success. Similarly with Magicka/INT enhancing magic.

Also, if you take your luck up really high -- into the thousands -- it can have a very significant effect on the quality of your loot, especially if the loot includes soul gems.